A system to measure the incorporation of lysine in vivo into collagen, elastin, and non-collagen proteins has been established. Spontaneously hypertensive rats have much greater rates of lysine incorporation into non-collagen protein of the mesenteric arteries than do normotensive control rats. This apparent increased synthesis of non-collagen protein appears to be controlled by the sympathetic nervous system, since denervating the vessels or treating the animals with a ganglionic blocker (hexamethonium) reduced in incorporation rates to the control level. Conversely, hydralazine which is a very effective hypotensive agent but actually causes an increase in sympathetic outflow did not reduce the rate of incorporation. Since these observations were made in very young rats it is suggested that increased protein syntheses in the small vessles may be one of the pathogenic factors in spontaneous hypertension.